Gastrointestinal anatomy and physiology of spotted wolffish (Anarhichas minor, Olafsen 1772) fed diets with increasing content of crab shell meal

The spotted wolffish (Anarhichas minor, Olafsen 1772) is a strong candidate for cold-water aquaculture, yet species specific nutrition requirements and knowledge of gut function remain limited. By-product like crab-shell meal which is rich in chitin and minerals, offers potential functional, cost and sustainability benefits as a feed ingredient. Feeds with 5% (Low-Crab) and 15% (High-Crab) crab-shell meal inclusion were formulated for this study to investigate the effects of increasing crab-shell inclusion on growth performance, nutrient digestibility, digestive enzyme activity, and distal intestinal morphology in spotted wolffish. A 6 months feeding trial was run prior to sampling. Crab-shell inclusion increased dietary ash and calcium and reduced crude protein as expected. Despite the reduction in dietary protein and energy that followed crab-shell inclusion, growth, survival, and condition remained similar among diets. Dry-matter digestibility decreased at 15% inclusion, but the digestibility of energy and amino acids stayed high, showing that the wolfish can grow on diets with a lower energy content. Increased dietary ash and calcium were not reflected in body, muscle, or liver mineral concentrations, indicating that excess minerals were excreted rather than accumulated. Enzyme activity was affected mainly by gastrointestinal region and feeding status. Chitinase activity was highest in the distal intestine, and trypsin activity was raised in the mid-gut, but neither enzyme showed consistent dietary responses. Freeze-dried and wet preparations resulted in comparable readings, meaning that sample preservation did not affect the measurements. Histological evaluation revealed normal intestinal architecture across all diets, with no signs of inflammation or structural damage. The present study demonstrates that crab-shell meal up to 15% can be included in spotted wolffish feed without affecting growth performance or overall gut health. Future research should focus on how crab inclusion influences feed intake, clarify the specific role of chitin in gastrointestinal function, and assess how excess dietary minerals may affect the long-term health of the digestive tract.